U.S. patent application number 11/712738 was filed with the patent office on 2007-09-06 for fuel cell.
This patent application is currently assigned to Honda Motor Co., Ltd.. Invention is credited to Takeshi Banba, Norimasa Kawagoe, Hideaki Kikuchi, Takashi Kosaka, Masaru Oda, Narutoshi Sugita, Yasuhiro Watanabe.
Application Number | 20070207372 11/712738 |
Document ID | / |
Family ID | 38329503 |
Filed Date | 2007-09-06 |
United States Patent
Application |
20070207372 |
Kind Code |
A1 |
Kikuchi; Hideaki ; et
al. |
September 6, 2007 |
Fuel cell
Abstract
A fuel gas flow field is formed on a surface of a rectangular
first metal separator. The fuel gas flow field includes flow
grooves extending in the direction of gravity. An outlet buffer is
provided at a lower end of the fuel gas flow field. The outlet
buffer includes an inclined surface inclined toward a fuel gas
discharge passage. The fuel gas discharge passage is positioned
below the outlet buffer. Outlet channel grooves are formed by
ridges provided between the fuel gas discharge passage and the
outlet buffer. Lower ends of the ridges are arranged in a zigzag
pattern.
Inventors: |
Kikuchi; Hideaki;
(Kawachi-gun, JP) ; Sugita; Narutoshi;
(Utsunomiya-shi, JP) ; Kawagoe; Norimasa;
(Utsunomiya-shi, JP) ; Oda; Masaru;
(Utsunomiya-shi, JP) ; Kosaka; Takashi;
(Utsunomiya-shi, JP) ; Banba; Takeshi;
(Shioya-gun, JP) ; Watanabe; Yasuhiro;
(Kawaguchi-shi, JP) |
Correspondence
Address: |
LAHIVE & COCKFIELD, LLP
ONE POST OFFICE SQUARE
BOSTON
MA
02109-2127
US
|
Assignee: |
Honda Motor Co., Ltd.
Tokyo
JP
|
Family ID: |
38329503 |
Appl. No.: |
11/712738 |
Filed: |
March 1, 2007 |
Current U.S.
Class: |
429/434 ;
429/483; 429/514 |
Current CPC
Class: |
H01M 8/1007 20160201;
H01M 8/241 20130101; H01M 8/0258 20130101; Y02E 60/50 20130101;
H01M 8/026 20130101; H01M 8/0267 20130101 |
Class at
Publication: |
429/38 ;
429/30 |
International
Class: |
H01M 8/02 20060101
H01M008/02; H01M 8/10 20060101 H01M008/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 2, 2006 |
JP |
2006-055735 |
Claims
1. A fuel cell formed by stacking a membrane electrode assembly and
a separator in a horizontal stacking direction, said membrane
electrode assembly including a pair of electrodes and an
electrolyte membrane interposed between said electrodes, said
separator having a rectangular shape including long sides extending
in the direction of gravity and short sides extending horizontally
in a direction perpendicular to the stacking direction, said
separator having a reactant gas flow field for supplying one of
reactant gases along an electrode surface in the direction of
gravity, wherein said reactant gas flow field includes an inlet
buffer at an upper position and an outlet buffer at a lower
position; a reactant gas supply passage for supplying the one of
reactant gases to said reactant gas flow field and a reactant gas
discharge passage for discharging the one of reactant gases from
the reactant gas flow field extend through said separator in the
stacking direction; and said reactant gas discharge passage is
positioned below said outlet buffer, and at least said outlet
buffer is inclined toward said reactant gas discharge passage.
2. A fuel cell according to claim 1, wherein said reactant gas
supply passage is positioned above said inlet buffer.
3. A fuel cell according to claim 1, wherein said reactant gas flow
field comprises a plurality of wavy flow grooves; and a coolant
flow field is provided for supplying a coolant horizontally to cool
the electrode surface.
4. A fuel cell according to claim 1, wherein a plurality of outlet
channel grooves are formed by ridges between said outlet buffer and
said reactant gas discharge passage; and lower ends of said ridges
are arranged in a zigzag pattern.
5. A fuel cell according to claim 4, wherein the lower end of said
ridge has a curved end surface.
6. A fuel cell according to claim 3, wherein said coolant flow
field is sandwiched between a plurality of said membrane electrode
assemblies.
7. A fuel cell according to claim 3, wherein said reactant gas flow
field comprises a plurality of wavy flow grooves and a plurality of
wavy ridges; and lower ends of said wavy ridges are arranged in a
zigzag pattern.
8. A fuel cell according to claim 7, wherein the lower end of said
wavy ridge has a curved end surface.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a fuel cell formed by
stacking a membrane electrode assembly and a separator in a
horizontal stacking direction. The membrane electrode assembly
includes a pair of electrodes and an electrolyte membrane
interposed between the electrodes. The separator has a rectangular
shape including long sides extending in the direction of gravity
and short sides extending horizontally in a direction perpendicular
to the stacking direction. The separator has a reactant gas flow
field for supplying one of reactant gases along an electrode
surface in the direction of gravity.
[0003] 2. Description of the Related Art
[0004] For example, a polymer electrolyte fuel cell employs a
membrane electrode assembly (MEA) which includes an anode, a
cathode, and an electrolyte membrane interposed between the anode
and the cathode. The electrolyte membrane is a solid polymer ion
exchange membrane. The membrane electrode assembly and separators
sandwiching the membrane electrode assembly make up a unit of a
power generation cell for generating electricity. In practical use
of such a fuel cell, normally, a predetermined numbers of power
generation cells are stacked together to form a fuel cell
stack.
[0005] In the fuel cell, a fuel gas flow field for supplying a fuel
gas to the anode, and an oxygen-containing gas flow field for
supplying an oxygen-containing gas to the cathode are formed in the
surfaces of the separator. Further, a coolant flow field as a
passage of a coolant is formed between the separators along the
surfaces of the separators.
[0006] In general, the fuel cell has internal manifold structure in
which fluid supply passages and fluid discharge passages extending
through the separators in the stacking direction are provided in
the fuel cell. The fuel gas, the oxygen-containing gas, and the
coolant as fluids are supplied to the fuel gas flow field, the
oxygen-containing gas flow field, and the coolant flow field
through the respective fluid supply passages, and then, discharged
into the fluid discharge passages.
[0007] As the fuel cell having the internal manifold structure, for
example, a fuel cell disclosed in Japanese Laid-Open Patent
Publication No. 6-20713 is known. As shown in FIG. 8, the fuel cell
includes a cell 1. The cell 1 has an air electrode 1b, a fuel
electrode 1c, and a solid polymer membrane 1a interposed between
the air electrode 1b and the fuel electrode 1c. Packings 2 are
attached on both left and right sides of the cell 1. Main surfaces
of the cell 1 and the packings 2 are sandwiched between a pair of
separators 3. The separators 3 have fuel gas supply grooves 4 on a
surface facing the fuel electrode 1c, and have oxygen-containing
gas supply grooves 5 on a surface facing the air electrode 1b. The
fuel gas supply grooves 4 and the oxygen-containing gas supply
grooves 5 extend vertically.
[0008] A fuel gas supply passage 6a and an oxygen-containing gas
supply passage 7a are provided at positions near upper corners of
the separator 3, and a fuel gas discharge passage 6b and an
oxygen-containing gas discharge passage 7b are provided at
positions near lower corners of the separator 3.
[0009] The fuel gas supplied from the fuel gas supply passage 6a
flows through the fuel gas supply grooves 4 downwardly in the
direction of gravity, and the oxygen-containing gas supplied from
the oxygen-containing gas supply passage 7a flows though the
oxygen-containing gas supply grooves 5 downwardly in the direction
of gravity.
[0010] However, in the conventional technique, the position of the
fuel gas discharge passage 6b is above the lower end of the fuel
gas supply grooves 4, and the position of the oxygen-containing gas
discharge passage 7b is above the lower end of the
oxygen-containing gas supply grooves 5. Therefore, water produced
in the power generation reaction tends to be retained at the lower
ends of the fuel gas supply grooves 4 and the oxygen-containing gas
supply grooves 5. In particular, when operation of the fuel cell is
stopped, and the fuel cell is exposed to the atmosphere at the
temperature below the freezing point, the retained water freezes.
Thus, due to expansion of the retained water, the fuel cell may be
damaged undesirably.
SUMMARY OF THE INVENTION
[0011] A main object of the present invention is to provide a fuel
cell having simple structure in which it is possible prevent water
produced in power generation reaction from being retained in the
fuel cell as much as possible, and the desired power generation
performance and the durability are maintained advantageously.
[0012] The present invention relates to a fuel cell formed by
stacking a membrane electrode assembly and a separator in a
horizontal stacking direction. The membrane electrode assembly
includes a pair of electrodes and an electrolyte membrane
interposed between the electrodes. The separator has a rectangular
shape including long sides extending in the direction of gravity
and short sides extending horizontally in a direction perpendicular
to the stacking direction. The separator has a reactant gas flow
field for supplying one of reactant gases along an electrode
surface in the direction of gravity.
[0013] The reactant gas flow field includes an inlet buffer at an
upper position and an outlet buffer at a lower position. A reactant
gas supply passage for supplying the one of reactant gases to the
reactant gas flow field and a reactant gas discharge passage for
discharging the reactant gas from the one of reactant gases flow
field extend through the separator in the stacking direction. The
reactant gas discharge passage is positioned below the outlet
buffer, and at least the outlet buffer is inclined toward the
reactant gas discharge passage.
[0014] In the structure, the reactant gas flow field comprises a
long side extending in the direction of gravity, and the reactant
gas flows through the reactant gas flow field in the direction of
gravity. Therefore, the water produced in the power generation
reaction is discharged smoothly by its own weight. The reactant gas
discharge passage is provided below the outlet buffer, and the
outlet buffer is inclined toward the reactant gas discharge
passage. Thus, the water produced in the power generation reaction
is not retained at the lower end of the reactant gas flow field,
and smoothly and reliably discharged into the reactant gas
discharge passage.
[0015] In particular, when operation is stopped, since the water
produced in the power generation reaction is discharged into the
reactant gas discharge passage by its own weight, the water is not
retained in the fuel cell. Thus, with the simple structure, it is
possible to prevent the damage of the fuel cell due to the frozen
water retained in the fuel cell.
[0016] The above and other objects, features and advantages of the
present invention will become more apparent from the following
description when taken in conjunction with the accompanying
drawings in which preferred embodiments of the present invention
are shown by way of illustrative example.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a partial exploded perspective view showing a fuel
cell stack according to a first embodiment of the present
invention;
[0018] FIG. 2 is partial cross sectional view showing the fuel cell
stack;
[0019] FIG. 3 is an exploded perspective view showing main
components of a unit cell of the fuel cell stack;
[0020] FIG. 4 is a front view showing a first metal separator of
the unit cell;
[0021] FIG. 5 is a front view showing a second metal separator of
the unit cell;
[0022] FIG. 6 is a cross sectional view showing a fuel cell stack
according to a second embodiment of the present invention;
[0023] FIG. 7 is a partial front view showing a first metal
separator of a fuel cell stack according to a third embodiment of
the present invention; and
[0024] FIG. 8 is a perspective view showing main components of a
conventional fuel cell.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] FIG. 1 is a partial exploded perspective view showing a fuel
cell stack (fuel cell) 10 according to a first embodiment of the
present invention. FIG. 2 is a partial cross sectional view showing
the fuel cell stack 10.
[0026] The fuel cell stack 10 includes a stack body 14 formed by
stacking a plurality of unit cells 12 in a substantially horizontal
direction indicated by an arrow A. At one end of the stack body 14
in the stacking direction, a terminal plate 16a is provided. An
insulating plate 18a is provided outside the terminal plate 16a,
and an end plate 20a is provided outside the insulating plate
18a.
[0027] At the other end of the stack body 14 in the stacking
direction, a terminal plate 16b is provided. An insulating plate
18b is provided outside the terminal plate 16b, and an end plate
20b is provided outside the insulating plate 18b (see FIG. 1). For
example, the fuel cell stack 10 is integrally held in a box-shaped
casing (not shown) including the end plates 20a, 20b having a
rectangular shape. Alternatively, components of the fuel cell stack
10 between the end plates 20a, 20b are integrally tightened by a
plurality of tie rods (not shown) extending in the direction
indicated by the arrow A.
[0028] A terminal 26a is provided at substantially the center of
the terminal plate 16a, and a terminal 26b is provided at
substantially the center of the terminal plate 16b. The terminals
26a, 26b are inserted into insulating cylinders 28 and extend
outwardly from the end plates 20a, 20b, respectively.
[0029] As shown in FIGS. 2 and 3, each of the unit cells 12
includes a membrane electrode assembly (electrolyte electrode
assembly) 30 and first and second metal separators 32, 34
sandwiching the membrane electrode assembly 30. The first and
second metal separators 32, 34 are thin metal plates fabricated to
have corrugated surfaces by press forming. Therefore, the first and
second metal separators 32, 34 have ridges and grooves in cross
section. Each of the first and second metal separators 32, 34 has a
rectangular shape including long sides oriented in the direction of
gravity indicated by an arrow C, and short sides oriented
horizontally in a direction indicated by an arrow B.
[0030] At an upper end of the unit cell 12 in the longitudinal
direction indicated by the arrow C in FIG. 3, an oxygen-containing
gas supply passage (reactant gas supply passage) 36a for supplying
an oxygen-containing gas, and a fuel gas supply passage (reactant
gas supply passage) 38a for supplying a fuel gas such as a
hydrogen-containing gas are provided. The oxygen-containing gas
supply passage 36a and the fuel gas supply passage 38a extend
through the unit cell 12 in the direction indicated by the arrow
A.
[0031] At a lower end of the unit cell 12 in the longitudinal
direction, a fuel gas discharge passage (reactant gas discharge
passage) 38b for discharging a fuel gas and an oxygen-containing
gas discharge passage (reactant gas discharge passage) 36b for
discharging the oxygen-containing gas are provided. The fuel gas
discharge passage 38b and the oxygen-containing gas discharge
passage 36b extend through the unit cell 12 in the direction
indicated by the arrow A.
[0032] At one end of the unit cell 12 in a lateral direction
indicated by the arrow B, a coolant supply passage 40a for
supplying a coolant is provided. At the other end of the unit cell
12 in the lateral direction, a coolant discharge passage 40b for
discharging the coolant is provided. The coolant supply passage 40a
and the coolant discharge passage 40b extend through the unit cell
12 in the direction indicated by the arrow A.
[0033] The membrane electrode assembly 30 includes an anode 44, a
cathode 46, and a solid polymer electrolyte membrane 42 interposed
between the anode 44 and the cathode 46. The solid polymer
electrolyte membrane 42 is formed by impregnating a thin membrane
of perfluorosulfonic acid with water, for example.
[0034] Each of the anode 44 and the cathode 46 has a gas diffusion
layer (not shown) such as a carbon paper, and an electrode catalyst
layer (not shown) of platinum alloy supported on porous carbon
particles. The carbon particles are deposited uniformly on the
surface of the gas diffusion layer. The electrode catalyst layer of
the anode 44 and the electrode catalyst layer of the cathode 46 are
fixed to both surfaces of the solid polymer electrolyte membrane
42, respectively.
[0035] The first metal separator 32 has a fuel gas flow field 48 on
its surface 32a facing the membrane electrode assembly 30. The fuel
gas flow field 48 is connected to the fuel gas supply passage 38a
at one end, and connected to the fuel gas discharge passage 38b at
the other end. As shown in FIGS. 3 and 4, the fuel gas flow field
48 includes a plurality of wavy flow grooves 48a extending in the
direction indicated by the arrow C. An inlet buffer 50a and an
outlet buffer 50b are provided at upper and lower ends of the wavy
flow grooves 48a. A plurality of bosses are provided in the inlet
buffer 50a and the outlet buffer 50b.
[0036] The inlet buffer 50a includes inclined surfaces 52a, 52b
inclined toward the fuel gas supply passage 38a and the
oxygen-containing gas supply passage 36a. The outlet buffer 50b
includes inclined surfaces 54a, 54b inclined toward the fuel gas
discharge passage 38b and the oxygen-containing gas discharge
passage 36b. The fuel gas supply passage 38a is provided above the
upper end of the inlet buffer 50a and the fuel gas discharge
passage 38b is provided below the lower end of the outlet buffer
50b.
[0037] A plurality of inlet channel grooves 56a are formed by a
plurality of ridges 58a provided between the fuel gas supply
passage 38a and the inlet buffer 50a. The inlet channel grooves 56a
are inclined toward the fuel gas supply passage 38a. Likewise, a
plurality of outlet channel grooves 56b are formed by a plurality
of ridges 58b provided between the fuel gas discharge passage 38b
and the outlet buffer 50b. The outlet channel grooves 56b are
inclined toward the fuel gas discharge passage 38b. Lower ends of
the ridges 58b are arranged in a zigzag pattern. Each of the lower
ends of the ridges 58b has a curved end surface (R-surface).
[0038] As shown in FIG. 5, the second metal separator 34 has an
oxygen-containing gas flow field 60 on its surface 34a facing the
membrane electrode assembly 30. The oxygen-containing gas flow
field 60 is connected to the oxygen-containing gas supply passage
36a at one end, and connected to the oxygen-containing gas
discharge passage 36b at the other end. The oxygen-containing gas
flow field 60 includes a plurality of wavy flow grooves 60a
extending in the direction indicated by the arrow C. An inlet
buffer 62a and an outlet buffer 62b are provided at upper and lower
ends of the wavy flow grooves 60a. A plurality of bosses are
provided in the inlet buffer 62a and the outlet buffer 62b.
[0039] The inlet buffer 62a includes inclined surfaces 64a, 64b
inclined toward the oxygen-containing gas supply passage 36a and
the fuel gas supply passage 38a. The outlet buffer 62b includes
inclined surfaces 66a, 66b inclined toward the oxygen-containing
gas discharge passage 36b and the fuel gas discharge passage 38b.
The oxygen-containing gas supply passage 36a is provided above the
upper end of the inlet buffer 62a and the oxygen-containing gas
discharge passage 36b is provided below the lower end of the outlet
buffer 62b.
[0040] A plurality of inlet channel grooves 68a are formed by a
plurality of ridges 70a provided between the oxygen-containing gas
supply passage 36a and the inlet buffer 62a. The inlet channel
grooves 68a are inclined toward the oxygen-containing gas supply
passage 36a. Likewise, a plurality of outlet channel grooves 68b
are formed by a plurality of ridges 70b provided between the
oxygen-containing gas discharge passage 36b and the outlet buffer
62b. The outlet channel grooves 68b are inclined toward the
oxygen-containing gas discharge passage 36b. Lower ends of the
ridges 70b are arranged in a zigzag pattern. Each of the lower ends
of the ridges 70b has a curved end surface (R-surface).
[0041] A coolant flow field 72 is formed between a surface 34b of
the second metal separator 34 and a surface 32b of the first metal
separator 32 (see FIG. 3). The coolant flow field 72 is connected
between the coolant supply passage 40a and the coolant discharge
passage 40b. The coolant flow field 72 is formed by stacking the
corrugated back surface of the fuel gas flow field 48 and the
corrugated back surface of the oxygen-containing gas flow field 60.
The grooves in the coolant flow field 72 extend in the direction
indicated by the arrow B.
[0042] A first seal member 74 is formed integrally on the surfaces
32a, 32b of the first metal separator 32 around the outer end of
the first metal separator 32. Further, a second seal member 76 is
formed integrally on the surfaces 34a, 34b of the second metal
separator 34 around the outer end of the second metal separator 34
(see FIG. 3).
[0043] In FIGS. 1 and 2, the insulating plates 18a, 18b are made of
insulating material such as polycarbonate (PC) or phenol resin. A
rectangular recess 80a is formed at the center of the insulating
plate 18a, and a rectangular recess 80b is formed at the center of
the insulating plate 18b. A hole 82a is formed at substantially the
center of the recess 80a, and a hole 82b is formed at substantially
the center of the recess 80b.
[0044] The terminal plates 16a, 16b are placed in the recesses 80a,
80b, respectively. The terminals 26a, 26b of the terminal plates
16a, 16b are inserted into the holes 82a, 82b through the
insulating cylinders 28, respectively. Holes 84a, 84b are formed
coaxially with the holes 82a, 82b at substantially the center of
the end plates 20a, 20b.
[0045] As shown in FIG. 2, seal members (e.g., gaskets 90) are
provided on the inner surfaces forming the oxygen-containing gas
supply passage 36a, the fuel gas supply passage 38a, the coolant
supply passage 40a, the oxygen-containing gas discharge passage
36b, the fuel gas discharge passage 38b, and the coolant discharge
passage 40b.
[0046] Next, operation of the fuel cell stack 10 will be described
below.
[0047] Firstly, as shown in FIG. 1, the oxygen-containing gas is
supplied to the oxygen-containing gas supply passage 36a of the end
plate 20a, and the fuel gas is supplied to the fuel gas supply
passage 38a of the end plate 20a. Further, the coolant such as pure
water or ethylene glycol is supplied to the coolant supply passage
40a of the end plate 20a. Thus, in the stack body 14, the
oxygen-containing gas, the fuel gas, and the coolant are supplied
to the unit cells 12 stacked in the direction indicated by the
arrow A.
[0048] As shown in FIGS. 3 and 5, the oxygen-containing gas from
the oxygen-containing gas supply passage 36a flows into the
oxygen-containing gas flow field 60 of the second metal separator
34, and flows along the cathode 46 of the membrane electrode
assembly 30.
[0049] At this time, as shown in FIG. 5, on the surface 34a of the
second metal separator 34, the oxygen-containing gas from the
oxygen-containing gas supply passage 36a flows through the inlet
channel grooves 68a formed between the ridges 70a, and is supplied
to the inlet buffer 62a. The oxygen-containing gas supplied to the
inlet buffer 62a is distributed separately in the direction
indicated by the arrow B, into the wavy flow grooves 60a of the
oxygen-containing gas flow field 60. Then, the oxygen-containing
gas flows through the wavy flow grooves 60a downwardly, along the
cathode 46 of the membrane electrode assembly 30.
[0050] As shown in FIGS. 3 and 4, on the surface 32a of the first
metal separator 32, the fuel gas from the fuel gas supply passage
38a flows through the inlet channel grooves 56a formed between the
ridges 58a, and is supplied to the inlet buffer 50a. The fuel gas
supplied to the inlet buffer 50a is distributed separately in the
direction indicated by the arrow B, into the wavy flow grooves 48a
of the fuel gas flow field 48. Then, the fuel gas flows through the
wavy flow grooves 48a downwardly, along the anode 44 of the
membrane electrode assembly 30.
[0051] Thus, in each of the membrane electrode assemblies 30, the
oxygen-containing gas supplied to the cathode 46, and the fuel gas
supplied to the anode 44 are consumed in the electrochemical
reactions at catalyst layers of the cathode 46 and the anode 44 for
generating electricity (see FIG. 2).
[0052] Then, as shown in FIG. 5, the oxygen-containing gas consumed
at the cathode 46 flows into the outlet buffer 62b connected to the
lower end of the oxygen-containing gas flow field 60. Further, the
oxygen-containing gas from the outlet buffer 62b flows through the
outlet channel grooves 68b formed between the ridges 70b into the
oxygen-containing gas discharge passage 36b.
[0053] Likewise, as shown in FIGS. 3 and 4, the fuel gas consumed
at the anode 44 flows into the outlet buffer 50b connected to the
lower end of the fuel gas flow field 48. Further, the fuel gas from
the outlet buffer 50b flows through the outlet channel grooves 56b
formed between the ridges 58b into the fuel gas discharge passage
38b.
[0054] Further, after the coolant flows from the coolant supply
passage 40a into the coolant flow field 72 between the first and
second metal separators 32, 34, the coolant flows in the horizontal
direction indicated by the arrow B. After the coolant cools the
membrane electrode assembly 30, the coolant is discharged into the
coolant discharge passage 40b.
[0055] In the first embodiment, for example, as shown in FIG. 5,
the second metal separator 34 has a rectangular shape, and the long
sides of the second metal separator 34 are oriented in the
direction of the gravity indicated by the arrow C. The
oxygen-containing gas flow field 60 comprises the wavy flow grooves
60a extending in the direction of gravity. In the structure, though
a relatively large amount of water is produced in the power
generation reaction, since the oxygen-containing gas flows in the
direction of gravity, the water produced in the power generation
reaction is reliably discharged downwardly along the wavy flow
grooves 60a by its own weight.
[0056] Further, the outlet buffer 62b is provided below the
oxygen-containing gas flow field 60, and the oxygen-containing gas
discharge passage 36b is provided below the outlet buffer 62b
through the outlet flow grooves 68b. Therefore, when the water
produced in the power generation reaction in the oxygen-containing
gas flow field 60 is discharged into the oxygen-containing gas
discharge passage 36b, the oxygen-containing gas is not retained in
the area below the oxygen-containing gas flow field 60 or in the
outlet buffer 62b.
[0057] The outlet buffer 62b has the inclined surface 66a inclined
toward the oxygen-containing gas discharge passage 36b. The outlet
flow grooves 68b inclined from the inclined surface 66a toward the
oxygen-containing gas discharge passage 36b are provided.
[0058] In the structure, the water produced in the power generation
reaction flows from the lower end of the oxygen-containing gas flow
field 60 to the outlet buffer 62b. The water flows smoothly into
the oxygen-containing gas discharge passage 36b through the outlet
channel grooves 68b while reliably preventing the water from being
retained undesirably.
[0059] In particular, when operation of the fuel cell stack 10 is
stopped, since the water produced in the oxygen-containing gas flow
field 60 is discharged into the oxygen-containing gas discharge
passage 36b by its own weight, the water is not retained in the
unit cell 12. Thus, it is possible to prevent the damage of the
unit cell 12 due to the frozen water retained in the unit cell
12.
[0060] Further, also in the fuel gas flow field 48 as shown in FIG.
4, the same advantages as in the case of the oxygen-containing gas
flow field 60 can be obtained. Moreover, in the other embodiments
as described later, the same advantages can be obtained.
[0061] Further, in the first embodiment, the oxygen-containing gas
flow field 60 includes the wavy flow grooves 60a. In the structure,
the length of the flow grooves 60a of the oxygen-containing gas
flow field 60 become large in comparison with the case of adopting
straight flow grooves. Thus, the pressure loss in the
oxygen-containing gas flow field 60 is increased, and the flow rate
of the oxygen-containing gas is increased. Accordingly, the water
is discharged from the oxygen-containing gas flow field 60
smoothly.
[0062] Further, since the oxygen-containing gas flows along the
wavy flow grooves 60a in the direction indicated by the arrow C,
the flow direction of the oxygen-containing gas changes in a wavy
manner. In the structure, the oxygen-containing gas is diffused
efficiently at the cathode 46, and improvement in the power
generation performance is achieved.
[0063] Further, the lower ends of the ridges 70b forming the outlet
channel grooves 68b are arranged in a zigzag pattern. Therefore,
the intervals between the lower end positions of the ridges 70b are
increased. In the structure, the water moving downwardly by its own
weight is not retained as water droplets, and reliably discharged
from the outlet channel grooves 68b. Since the lower end of the
ridge 70b has a curved surface (R-surface), it is possible to
further reliably prevent the water droplets from being kept at the
lower end of the ridge 70b.
[0064] FIG. 6 is a partial cross sectional view showing a fuel cell
stack (fuel cell) 100 according to a second embodiment of the
present invention. The constituent elements that are identical to
those of the fuel cell stack 10 according to the first embodiment
are labeled with the same reference numeral, and description
thereof will be omitted.
[0065] The fuel cell stack 100 is formed by stacking a plurality of
cell units 102 in a horizontal direction. Each of the cell units
102 is formed by stacking a first metal separator 32, a first
membrane electrode assembly 30a, a third metal separator 104, a
second membrane electrode assembly 30b, and a second metal
separator 34 in a direction indicated by an arrow A. The first and
second membrane electrode assemblies 30a, 30b have the same
structure as that of the membrane electrode assembly 30.
[0066] The third metal separator 104 has an oxygen-containing gas
flow field 60 (having the same structure as that shown in FIG. 5)
on its surface facing the first membrane electrode assembly 30a,
and a fuel gas flow field 48 (having the same structure as that
shown in FIG. 4) on its surface facing the second membrane
electrode assembly 30b.
[0067] In the second embodiment, no coolant flow field 72 is
provided between the first and second membrane electrode assemblies
30a, 30b, and the so-called skipping cooling structure is adopted.
Thus, the same effect as in the case of the first embodiment can be
obtained. For example, the overall size of the fuel cell stack 100
in the stacking direction is reduced effectively.
[0068] FIG. 7 is a partial front view showing a first metal
separator 110 of a fuel cell stack according to a third embodiment
of the present invention. The constituent elements that are
identical to those of the first metal separator 32 according to the
first embodiment are labeled with the same reference numeral, and
description thereof will be omitted.
[0069] The first metal separator 110 has a fuel gas flow field 112
on its surface 110a facing the membrane electrode assembly 30. The
fuel gas flow field 112 includes a plurality of wavy ridges 112a
and a plurality of wavy flow grooves 112b alternately. The lower
ends of the wavy ridges 112a are arranged in a zigzag pattern. Each
of the lower ends of the wavy ridges 112a has a curved end surface
(R-surface).
[0070] Though not shown, the second metal separator has the same
structure as that of the first metal separator 110.
[0071] In the third embodiment, the fuel gas flow field 112 is
formed by the wavy ridges 112a, and the lower ends of the wavy
ridges 112a are arranged in a zigzag pattern. Therefore, when the
water produced in the power generation reaction moves downwardly
along the wavy ridges 112a by its own weight, the water is not
retained in the fuel gas flow field 112 as water droplets, and the
water is smoothly discharged into the outlet buffer 50b. The lower
end of the wavy ridge 112a has a curved end surface (R-surface). In
the structure, it is possible to further reliably prevent the water
droplets from being kept at the lower end of the wavy ridge
112a.
[0072] While the invention has been particularly shown and
described with reference to preferred embodiments, it will be
understood that variations and modifications can be effected
thereto by those skilled in the art without departing from the
spirit and scope of the invention as defined by the appended
claims.
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